DC-voltage losses over sliprings????

[Parcifal]

Hello,
I am currently working on a project which includes a long electrical/optical cable (1100m) which is wound onto a large winch. This cable will be used on a ship to tow a data-acquisition box (geotechnical research).
Now the electrical wires are connected to a slipring-box and hence we can connect our power and signal wires without risking to have spaghetti-situations.
Now following problem is bugging me :
I connect 12VDC over 2 terminals of the cable and measure the output : 12 VDC. This looks ok. (resistance of 1 wire of the cable is 40 Ohm).
Now next thing I do : I connect a load (acquisition tool) with an internal resistance of 65 KOhm, rated to operate on 12VDC. I apply the power and measure 4.3 VDC on the output-terminals of the cable!!!!!.
I am puzzled but not defeated : I vary the input voltage in small steps from 5 to 16 VDC. Result : The measured voltage remains 4.3 VDC (+- 0.1 VDC).
Does anybody have any clue which might be causing this phenomenon? Only thing I can imagine is there is a voltage drop over the slip-rings of the winch which is current-related. Any (argumented) guess is welcome.
Regards,
Puzzled Parcifal.

 

[busbar]

PP-- have you measured voltage drop directly across the brush/slipring assembly? Does the circuit use two slipring/brush assembiles or {one brush and} the winch frame for one of the 12V connections?

 

[teco]

parcifal:- try looking at the brush grade on the sliprings. had a simular problem and changed drush grade to a silver graphite type and this fixed the problem.
kind regds.